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std: remove loop from growCapacity

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I measured this against master branch and found no statistical
difference. Since this code is simpler and logically superior due to
always leaving sufficient unused capacity when growing, it is preferred
over status quo.
This commit is contained in:
Andrew Kelley 2025-09-19 17:48:07 -07:00
parent 4d1b15bd9d
commit 0c1fbc4ea6
4 changed files with 17 additions and 39 deletions
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2
lib/std/Io/Writer.zig
View file

@ -2661,7 +2661,7 @@ pub const Allocating = struct {
pub fn ensureTotalCapacity(a: *Allocating, new_capacity: usize) Allocator.Error!void {
// Protects growing unnecessarily since better_capacity will be larger.
if (a.writer.buffer.len >= new_capacity) return;
const better_capacity = ArrayList(u8).growCapacity(a.writer.buffer.len, new_capacity);
const better_capacity = ArrayList(u8).growCapacity(new_capacity);
return ensureTotalCapacityPrecise(a, better_capacity);
}

17
lib/std/array_list.zig
View file

@ -172,7 +172,7 @@ pub fn AlignedManaged(comptime T: type, comptime alignment: ?mem.Alignment) type
// a new buffer and doing our own copy. With a realloc() call,
// the allocator implementation would pointlessly copy our
// extra capacity.
const new_capacity = Aligned(T, alignment).growCapacity(self.capacity, new_len);
const new_capacity = Aligned(T, alignment).growCapacity(new_len);
const old_memory = self.allocatedSlice();
if (self.allocator.remap(old_memory, new_capacity)) |new_memory| {
self.items.ptr = new_memory.ptr;
@ -408,7 +408,7 @@ pub fn AlignedManaged(comptime T: type, comptime alignment: ?mem.Alignment) type
// Protects growing unnecessarily since better_capacity will be larger.
if (self.capacity >= new_capacity) return;
const better_capacity = Aligned(T, alignment).growCapacity(self.capacity, new_capacity);
const better_capacity = Aligned(T, alignment).growCapacity(new_capacity);
return self.ensureTotalCapacityPrecise(better_capacity);
}
@ -1160,7 +1160,7 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type {
/// Invalidates element pointers if additional memory is needed.
pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
if (self.capacity >= new_capacity) return;
return self.ensureTotalCapacityPrecise(gpa, growCapacity(self.capacity, new_capacity));
return self.ensureTotalCapacityPrecise(gpa, growCapacity(new_capacity));
}
/// If the current capacity is less than `new_capacity`, this function will
@ -1359,17 +1359,12 @@ pub fn Aligned(comptime T: type, comptime alignment: ?mem.Alignment) type {
return self.getLast();
}
const init_capacity = @as(comptime_int, @max(1, std.atomic.cache_line / @sizeOf(T)));
const init_capacity: comptime_int = @max(1, std.atomic.cache_line / @sizeOf(T));
/// Called when memory growth is necessary. Returns a capacity larger than
/// minimum that grows super-linearly.
pub fn growCapacity(current: usize, minimum: usize) usize {
var new = current;
while (true) {
new +|= new / 2 + init_capacity;
if (new >= minimum)
return new;
}
pub fn growCapacity(minimum: usize) usize {
return minimum +| (minimum / 2 + init_capacity);
}
};
}

14
lib/std/deque.zig
View file

@ -56,7 +56,7 @@ pub fn Deque(comptime T: type) type {
/// Invalidates element pointers if additional memory is needed.
pub fn ensureTotalCapacity(deque: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
if (deque.buffer.len >= new_capacity) return;
return deque.ensureTotalCapacityPrecise(gpa, growCapacity(deque.buffer.len, new_capacity));
return deque.ensureTotalCapacityPrecise(gpa, std.ArrayList(T).growCapacity(new_capacity));
}
/// If the current capacity is less than `new_capacity`, this function will
@ -243,18 +243,6 @@ pub fn Deque(comptime T: type) type {
return index - head_len;
}
}
const init_capacity: comptime_int = @max(1, std.atomic.cache_line / @sizeOf(T));
/// Called when memory growth is necessary. Returns a capacity larger than
/// minimum that grows super-linearly.
fn growCapacity(current: usize, minimum: usize) usize {
var new = current;
while (true) {
new +|= new / 2 + init_capacity;
if (new >= minimum) return new;
}
}
};
}

23
lib/std/multi_array_list.zig
View file

@ -457,24 +457,19 @@ pub fn MultiArrayList(comptime T: type) type {
/// Invalidates element pointers if additional memory is needed.
pub fn ensureTotalCapacity(self: *Self, gpa: Allocator, new_capacity: usize) Allocator.Error!void {
if (self.capacity >= new_capacity) return;
return self.setCapacity(gpa, growCapacity(self.capacity, new_capacity));
return self.setCapacity(gpa, growCapacity(new_capacity));
}
const init_capacity = init: {
var max = 1;
for (fields) |field| max = @as(comptime_int, @max(max, @sizeOf(field.type)));
break :init @as(comptime_int, @max(1, std.atomic.cache_line / max));
const init_capacity: comptime_int = init: {
var max: comptime_int = 1;
for (fields) |field| max = @max(max, @sizeOf(field.type));
break :init @max(1, std.atomic.cache_line / max);
};
/// Called when memory growth is necessary. Returns a capacity larger than
/// minimum that grows super-linearly.
fn growCapacity(current: usize, minimum: usize) usize {
var new = current;
while (true) {
new +|= new / 2 + init_capacity;
if (new >= minimum)
return new;
}
/// Given a lower bound of required memory capacity, returns a larger value
/// with super-linear growth.
pub fn growCapacity(minimum: usize) usize {
return minimum +| (minimum / 2 + init_capacity);
}
/// Modify the array so that it can hold at least `additional_count` **more** items.